In the past thermoelectric heat pumps used in certain military applications have been exposed to temperatures ranging from about 60 degrees Centigrade (Celsius) to about 251 degrees Celsius for periods of one year or more. As a result the performance of the thermoelectric heat pumps were found to decrease about ten percent per year. This degradation in performance could not be tolerated as it thwarted operation of the military devices incorporating the thermoelectric heat pumps.
The heat pumps being used in the military devices were commercial grade heat pumps have a structure soldered together using the typical bismuth-tin composition solder. Examination of failed thermoelectric heat pumps under a high power microscope revealed that the tin of the bismuth-tin solder had separated with the tin forming a film over the surface of the thermoelectric material adjacent its soldered ends.
Further experimentation revealed that the tin film acted as a resistor connected across the thermoelectric material causing an IR drop or a partial short in the thermoelectric material. The value of the IR drop increased with the increasing length and thickness of the tin film; the length and thickness increased as a function of time.
Further, in commercial devices using lead-tin solder, it was found that at elevated temperatures the lead-tin solder reacted with the thermoelectric material to form a region of poor thermoelectric characteristics.
Thus the advantage of the present invention over the prior art devices is the elimination of the tin film causing the IR drop or partial short of the thermoelectric material.
Another advantage of the invention is the provision of a solder which does not react with the thermoelectric material.
A further advantage of the invention is the provision of a thermoelectric device capable of withstanding temperatures up to about 251 degrees Celsius.